Diesel engines typically are provided with exhaust aftertreatment devices to clean exhaust gases by trapping nitrogen oxides (NOx) and carbon particulates formed during engine combustion. Traps for these contaminants, however, become sated with engine use and must be regenerated periodically.
It is known to employ a hydrocarbon reformer in communication with the diesel exhaust stream to generate hydrogen on command by catalytic partial reduction of diesel fuel. Hydrogen is an excellent reductant for regeneration of NOx and particulate traps.
Hydrogen-containing reformate is produced by injecting diesel fuel and air into a mixing chamber in the reformer ahead of the catalyst. Typically, such mixture at start-up is substantially leaner in fuel than the mixture used for reforming after the catalyst reaches operating temperature. The initial mixture in the mixing chamber is ignited to start combustion, providing a hot exhaust that rapidly warms the catalyst, after which combustion is terminated: the flame is quenched, and the mixture is switched to a rich condition for reforming.
During the reforming process, the igniter remains off, waiting within the mixing chamber for use in the next combusting ignition cycle. In the prior art, a spark plug is used to ignite the mixture. While not in sparking mode, during reforming mode the spark plug points can become progressively fouled with carbon deposits on the plug tip and ceramic sleeve, forming an electric short and inhibiting or outright preventing sparking when subsequently needed.
Various solutions have been proposed to counter such carbon buildup, for example, using a high energy ignition coil to generate a strong current to deliver intense heat in a short time and thus to burn off any carbon deposits prior to the start of sparking. This solution has not been operationally satisfactory and further it requires addition of a high energy coil and controls, thus increasing manufacturing cost and complexity.
Alternatively, it has been proposed to use a glow plug in place of the sparking plug; however, glow plugs generically are slow to heat and thus are not well suited to this application. Recently, several manufacturers have invented a very rapid heating diesel engine glow plug, such as a ceramic glow plug, that would permit use of a glow plug in the present diesel exhaust reformer application except that the rapid-start glow plug lacks the self-regulating characteristics of a convention glow plug and is easily damaged or destroyed by overheating without control. The new glow plugs have a heat-up time of only one to two seconds and require significantly less energy than conventional glow plugs. See, for example, BERU-assigned U.S. Pat. Nos. 6,335,516 and 7,160,584.
What is needed in the art is a combustion ignition control strategy and method that permits use of a rapid-start glow plug with a 12 volt source while preventing damage or destruction of the glow plug.
It is a principal object of the present invention to employ a rapid-start glow plug as a reformer igniter in a hydrocarbon reformer of a diesel engine emissions regeneration system.